This invention relates to telescopic gunsights (sometimes referred to hereinafter merely as "scopes" or "sights"), to reticles useful therein, and to firearms equipped with telescopic sights. The scopes of this invention are discussed herein principally with reference to their use on rifles. It will become apparent, however, that they also may be used on other devices capable of propelling projectiles along substantially predeterminable trajectories, e.g., handguns, crossbows, longbows and artillery.
A factor which must be taken into account in long-range shooting is the curved trajectory traversed by a bullet as it falls below the axis of the rifle bore while traveling the distance from the rifle muzzle to the target, i.e., "range." An aiming line of sight emanating from a reticle aiming point of a scope rigidly affixed to a rifle is straight, and hence the line of sight can intersect the curved trajectory only at discrete ranges. At ranges other than these the bullet will pass below or above the aiming line of sight, necessitating the use of "hold-over" or "hold-under" in aiming. There have been proposed in the prior art numerous reticles and scopes designed to provide the shooter with information concerning the approximate range to a target and/or to provide the shooter with a plurality of aiming points for shooting at targets at various ranges, i.e., aiming points producing line of sight/trajectory intersections at various ranges. (See Gunsight Guide [Stoeger Arms Corp. 1968], pages 165-172 for a discussion of some such reticles and scopes.)
There are very accurate range-finder devices which function on a triangulation principle. Most scopes, however, are not large enough to accommodate this sort of system. As a consequence, most range-finding capability found in prior art scopes is based on the principle that if the angle subtended by a target of known or estimatable size can be determined, then the range can be calculated, or at least approximated. Such scopes therefore have included reticle features such as dots, cross hair pairs, apertures or posts subtending known angles. By comparing the apparent size of one of these features to the image of a target of known or estimatable height viewed through the scope, the range of the target can be estimated. Thus, if a dot subtending six inches at a range of 100 yds. (i.e., subtending 6 "minutes of angle," as that expression is used herein and in the appended claims) subtends only 2/3 of a target estimated to be 18 inches high, i.e., subtends 12 inches, then the estimated range can be calculated: Estimated Range = (12 inches / 6 inches) .times. 100 yds. = 200 yds.
Range-finding capability alone is insufficient to achieve proper aiming. The shooter usually must also determine the correct hold-over for the estimated range and estimate the aiming sight picture that will provide the correct hold-over. This usually requires reference to memorized or recorded bullet trajectory information, and requires mental calculations to convert the hold-over to units of the target's height for reference in establishing the proper aiming sight picture. Some prior art scopes provide a plurality of range-related aiming points intended to assist the shooter in estimating the appropriate aiming sight picture for various ranges.
Exemplary of specific prior art scopes of the types just described are those disclosed in U.S. Pat. No.'s 3,190,003; 1,190,121; 3,392,450; 3,431,652; and 3,492,733. The last four of these patents propose scopes providing a plurality of range-related aiming points accompanied with aiming point selection means, the use of which depends on relative height of the image of a target of known or estimatable height compared to the height of a feature in the reticle. Scopes of this general type are sometimes referred to hereinafter as "point-of-aim" scopes.